1. Field of the Invention (Technical Field)
The present invention relates to modified inorganic coatings and methods for producing and using such coatings, particularly useful as chemical sensors. These coatings provide a combination of both chemical and steric selectivity to select among various chemical species.
2. Background Art
Determining and/or monitoring the presence of certain chemical species within an environment, e.g., pollutants, toxic substances, and other predetermined compounds, is becoming of increasing importance with respect to such fields as health, environmental protection, resource conservation, and chemical processes.
There exist very sophisticated and complicated systems which are capable of detecting the presence of, for example, a substance in the atmosphere, even down to as low a level as a trillionth of a gram. However, many devices are impractical for field applications. For example, in analyzing water or soil samples for the presence of harmful substances, the samples are generally collected from the field and then taken to the lab and subjected to analysis using, for example, a gas or liquid chromatograph and/or a mass spectrometer. These types of analysis equipment, while very sophisticated and precise, are not practical for use in the field, require a substantial capital investment, and often take a long period of time for completion of the analysis, i.e., often up to several days.
Chemical and biochemical sensors, which are less expensive and smaller in size than those discussed above, also can provide for determining and/or monitoring the presence of certain chemical species in an environment. Many chemical sensors consist of a thin coating (e.g., a polymer film) which selectively sorbs the chemical species of interest and a sensing device to detect this sorption and convert it into a monitorable signal. A review of coating materials used in this way with surface acoustic wave sensors is given in M. S. Nieuwenhuizen, et al., "Surface Acoustic Wave Chemical Sensors," Sensors and Materials, Vol. 1, pp. 261-300 (1989). The sensitivity of the sensor is determined by the sensitivity of the detecting device to the perturbation in film properties and by the total sorption capacity of the coating.
One class of chemical sensor is generally known as a piezoelectric sensor, such as surface acoustic wave (SAW), acoustic plate mode (APM), or quartz crystal microbalance (QCM) devices. These sensors are based on a piezoelectric substrate and are also called gravimetric sensors, due to their ability to detect a change in mass in a coating formed on the device surface. SAW sensors are discussed in detail in M. S. Nieuwenhuizen, et al., Ibid. By employing an alternating voltage to an interdigital transducer formed on the piezoelectric crystal, there results a surface acoustic wave. The propagation velocity of this surface acoustic wave is a sensitive probe of changes in the properties of the coating material. Coating properties which are known to elicit a detectable SAW sensor response are mass (i.e., as determined by the thickness and density of the coating), elasticity, viscoelasticity, conductivity, and dielectric constant (G. C. Frye and S. J. Martin, "Materials Characterization Using Surface Acoustic Wave Devices," E. G. Brame, Jr., Editor, Applied Spectroscopy Reviews (1991)). Changes in these properties can also result in changes in the attenuation (i.e., loss of acoustic power) of the wave. In some situations, monitoring attenuation may be preferable to monitoring velocity or, alternatively, it has been found that there are some situations where simultaneously monitoring both velocity and attenuation can be useful (G. C. Frye and S. J. Martin, "Dual Output Surface Acoustic Wave Sensors for Molecular Identification," Sensors and Materials, Vol. 2, pp. 187-195 (1991)). Thus, when a substance sorbs into a coating formed on the surface of a SAW device, there is produced a response. To indicate their sensitivity, SAW devices are capable of detecting mass changes as low as about 100 pg/cm.sup.2. APM devices, discussed in detail in S. J. Martin, et al., Sensors and Actuators, "Characterization of SH Acoustic Plate Mode Liquid Sensors," Vol. 20, pp. 253-268 (1989), are similar to SAW devices except the acoustic wave used can be operated with the device in contact with a liquid. Similarly, an alternating voltage applied to the two opposite electrodes on a QCM (typically AT-cut quartz) device induces a thickness shear mode whose resonance frequency changes in proportion to mass changes in a coating material. However, while these piezoelectric devices are very sensitive detectors, especially for mass, they are not inherently selective with respect to different substances.
There are a variety of other chemical sensors, such as optical sensors, where a sensor response is produced when there is a change in the properties of the sensor coating.
Sensor selectivity, the ability to detect a chemical species in an environment containing other chemical species, is generally determined solely by the ability of the coating to specifically sorb the species to be detected to the exclusion of all others. For most coatings, some selectivity is obtained based solely on providing stronger chemical interactions between the coating and the target species than occur between the coating and the species which are not to be detected. (M. S. Nieuwenhuizen, et al., Ibid.)
Several references, discussed below, disclose porosity and/or chemical sorption; however, none of these references relate to the combination of controlled porosity and chemical selectivity.
U.S. Pat. No. 3,164,004, to King, Jr, entitled Coated Piezoelectric Analyzers, discloses a variety of coating materials for use with the piezoelectric sensors typically called quartz crystal microbalances. Included in these coating materials are a few porous materials: silica gel, alumina, and molecular sieves. These materials are listed simply as sorbents for water, and the use of controlled pore size for discrimination is not mentioned. In addition, as described, these materials do not have tailored surface chemistry for providing chemical selectivity.
U.S. Pat. No. 4,847,594, to Stetter, entitled Sensor for Detecting the Exhaustion of an Adsorbent Bed, relates to a chemiresistor for use in monitoring breakthrough of a gas mask adsorbent bed. The sensor uses a vapor adsorbing coating which consists of carbon embedded in a silicone film. Although porous, this carbon does not have a tailored surface chemistry, nor is the porosity used as a method for obtaining selectivity.
U.S. Pat. No. 4,834,496, to Blyler, Jr., et al., entitled Optical Fiber Sensors for Chemical Detection, discloses an optical fiber sensor which uses coatings consisting of optically active species (e.g., fluorescent dyes) contained in a permeable polymer coating around the fiber.
U.S. Pat. No. 4,504,522, to Kaiser, et al., entitled Method of Making a Titanium Dioxide Oxygen Sensor Element by Chemical Vapor Deposition, describes a chemical vapor deposition technique for titanium dioxide films to be used as automotive oxygen sensors. These films have porosity; however, the scale of this porosity is much too large to provide size exclusion. Oxygen is sensed due to its reaction with the film material rather than any selective adsorption by the material surfaces.
U.S. Pat. No. 4,277,322, to Kane, entitled Oxygen Sensor, discloses the use of a porous ceramic filler to protect a platinum sensor element. The porosity is not used for selectivity.
Parent application Ser. No. 07/580,373 now U.S. Pat. No. 5,151,110 relates to steric based selectivity using a different type of coating containing small crystals of zeolites embedded in a dense silicate matrix. These zeolites have a crystalline pore structure with uniform sized openings. Molecules smaller than the openings showed large sensor responses due to sorption on the surfaces of the zeolite pores while species too large to have access to the pores due to their larger size showed a much smaller sensor response (also see T. Bein, et al., "Molecular Sieve Sensors for Selective Detection at the Nanogram Level," Journal of the American Chemical Society, Vol. 111, pp. 7640-41 (1989)).